1. Active filters are electronic, while passive filters are mechanical;

2. The active filter detects a certain harmonic order and then cancels it. The passive filter forms a certain harmonic channel through the cooperation of the reactor and the capacitor to absorb the harmonics;

3. Use passive filters because of capacitors.

For the surface acoustic filter, its two input pins 1 and 2, its two output pins 3 and 4, and the input and output pins are insulated (as shown in the figure), and they are all insulated from the shielding pin 5. Therefore, when measuring the resistance between two using the R×1 k Ω range of a multimeter, it should be ∞. If there is a resistance of hundreds or thousands of ohms, it indicates that the performance of the surface acoustic filter has deteriorated. If the resistance is very small, it indicates that the surface acoustic filter has broken down.